Method of driving a laser diode

ABSTRACT

An ultrashort pulse/ultra-high power laser diode with a simple structure and configuration. The laser diode can be driven by a pulse current which is 10 or more times higher than a threshold current value. The width of the pulse current is preferably 10 nanoseconds or less, and the value of the pulse current is specifically 0.4 amperes or over.

RELATED APPLICATION DATA

This application is a division of U.S. patent application Ser. No.12/506,713, filed Jul. 21, 2009, the entirety of which is incorporatedherein by reference to the extent permitted by law. The presentapplication claims the benefit of priority to Japanese PatentApplication No. JP 2008-194373 filed in the Japan Patent Office on Jul.29, 2008, the entirety of which is incorporated by reference herein tothe extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of driving a laser diode.

2. Description of the Related Art

Recently, for researches in a leading-edge science region using laserlight with a pulse duration in the attosecond range or the femtosecondrange, ultrashort pulse/ultra-high power lasers have been frequentlyused. As the ultrashort pulse/ultra-high power laser, for example, atitanium/sapphire laser is known. However, the titanium/sapphire laseris an expensive and large solid laser light source, which is a mainimpediment to the spread of technology. If the ultrashortpulse/ultra-high power laser is realized through the use of a laserdiode, a large reduction in size and price of the ultrashortpulse/ultra-high power laser and high stability of the ultrashortpulse/ultra-high power laser are achieved.

On the other hand, in the communications field, a reduction in pulsewidths of laser diodes has been actively studied since 1960's. As amethod of generating a short pulse in a laser diode, a gain switchingmethod, a loss switching method (a Q-switching method) and a modelocking method are known, and in these method, a laser diode is combinedwith a semiconductor amplifier, a nonlinear optical element, an opticalfiber or the like to obtain higher power.

SUMMARY OF THE INVENTION

In the gain switching method which is the easiest method among theabove-described methods, when a laser diode is driven by a short pulsecurrent, a light pulse with a pulse width of approximately 20picoseconds to 100 picoseconds is generated as described in J. Ohya etal., Appl. Phys. Lett. 56 (1990) 56., J. AuYeung et al., Appl. Phys.Lett. 38 (1981) 308., N. Yamada et al., Appl. Phys. Lett. 63 (1993)583., J. E. Ripper et al., Appl. Phys. Lett. 12 (1968) 365., and“Ultrafast diode lasers”, P. Vasil'ev, Artech House Inc., 1995. In thegain switching method, it is only necessary to drive a commerciallyavailable laser diode by a short pulse current, so a short-pulse lightsource with a pulse duration in the picosecond range is achieved with anextremely simple device configuration. However, the peak power of alight pulse is approximately 0.1 watts to 1 watt in an 850-nm-wavelengthAlGaAs-based laser diode, and approximately 10 milliwatts to 100milliwatts in a 1.5-μm-wavelength InGaAsP-based laser diode. Therefore,for example, as a light source needing high peak power used fortwo-photon absorption, the light powers of the above laser diodes arenot sufficient. To increase the peak power, a complicated and difficultconfiguration formed by a combination of the mode locking method and asemiconductor amplifier or an optical fiber amplifier is necessary.

Thus, there have been few reported examples of a laser diode apparatusaiming at higher power on the basis of “an all-semiconductor structure”which is an essential condition for a ultimate reduction in size, thatis, a laser diode apparatus configured of only a laser diode or only acombination of a laser diode and a semiconductor device, specifically alaser diode apparatus configured of a 405-nm-wavelength laser diodewhich is made of a GaN-based compound semiconductor. Therefore, when an“all-semiconductor” pulse laser having a high peak power at a wavelengthof 405 nm is achieved, the pulse laser may be used as a light source ofa volumetric optical disk system which is expected as a next-generationoptical disk system following a Blu-ray optical disk system, and aconvenient ultrashort pulse/ultra-high power light source covering theentire wavelength band of a visible light range may be achieved by thepulse laser, thereby a light source necessary in the medical field, thebio imaging field or the like may be provided.

It is desirable to provide and a method of driving an ultrashortpulse/ultra-high power laser diode with a simple structure andconfiguration.

According to a first embodiment of the invention, there is provided amethod of driving a laser diode, the laser diode being driven by a pulsecurrent which is 10 or more times, preferably 20 or more times, morepreferably 50 or more times higher than a threshold current valueI_(th).

In this case, the threshold current value I_(th) indicates a currentflowing through a laser diode when laser oscillation starts, and anafter-mentioned threshold voltage value V_(th) indicates a voltageapplied to the laser diode at this time, and a relationship ofV_(th)=R×I_(th)+V₀ is established where the internal resistance of thelaser diode is R (Ω). In this case, V₀ is a built-in potential of a p-njunction.

According to a second embodiment of the invention, there is provided amethod of driving a laser diode, the laser diode being driven by a pulsevoltage which is 2 or more times, preferably 4 or more times, morepreferably 10 or more times higher than the threshold voltage valueV_(th).

In the method of driving a laser diode according to the first embodimentof the invention (hereinafter referred to as “the first embodiment ofthe invention” in some cases), a mode in which the width of the pulsecurrent is 10 nanoseconds or less, preferably 2 nanoseconds or less maybe applied. Moreover, in the first embodiment of the invention includingsuch a preferable mode, a mode in which the value of the pulse currentis 0.4 amperes or over, preferably 0.8 amperes or over may be applied.Alternatively, a mode in which the value of the pulse current is 3.5×10⁴ampere/cm² or over, preferably 7×10⁴ ampere/cm² or over in terms of thevalue of the pulse current per cm² of the active layer (per cm² of ajunction region area), that is, in terms of current density (which isoperation current density in ampere/cm²) may be applied. The lower limitof the width of the pulse current depends on specifications of the pulsegenerator, or the like. The upper limit of the value of the pulsecurrent may be determined on the basis of the specifications of a usedlaser diode.

In the method of driving a laser diode according to the secondembodiment of the invention (hereinafter referred to as “the secondembodiment of the invention” in some cases), a mode in which the widthof the pulse voltage is 10 nanoseconds or less, preferably 2 nanosecondsor less may be applied. Moreover, in the second embodiment of theinvention including such a preferable mode, a mode in which the value ofthe pulse voltage is 8 volts or over, preferably 16 volts or over may beapplied. The lower limit of the width of the pulse voltage depends onthe specifications of the pulse generator, or the like. The upper limitof the value of the pulse voltage may be determined on the basis of thespecifications of a used laser diode.

In the first embodiment of the invention and the second embodiment ofthe invention which includes various preferable modes described above(hereinafter simply collectively referred to as “the invention” in somecases), a mode in which the laser diode is a laser diode having a ridgestripe type separated confinement heterostructure (an SCH structure) maybe applied. A ridge section is formed by removing a part of anafter-mentioned second compound semiconductor layer in a thicknessdirection by, for example, an RIE method.

In the invention including the above-described preferable mode, thelaser diode may include a laminate structure body including a firstcompound semiconductor layer, an active layer having a quantum wellstructure and the second compound semiconductor layer, a first electrodeelectrically connected to the first compound semiconductor layer, and asecond electrode electrically connected to the second compoundsemiconductor layer, and the laminate structure body may be made of anAlGaInN-based compound semiconductor, that is, the laser diode may be aGaN-based laser diode.

In this case, specific examples of the AlGaInN-based compoundsemiconductor may include GaN, AlGaN, GaInN and AlGaInN. Moreover, ifnecessary, a boron (B) atom, a thallium (Tl) atom, an arsenic (As) atom,a phosphorus (P) atom or an antimony (Sb) atom may be included in thesecompound semiconductors. Further, the active layer having the quantumwell structure has a structure in which at least one well layer and atleast one barrier layer are laminated, and examples of a combination of(a compound semiconductor forming the well layer, a compoundsemiconductor forming the barrier layer) may include (In_(y)Ga_((1-y))N,GaN), (In_(y)Ga_((1-y))N, In_(z)Ga_((1-z))N) [y>z], and(In_(y)Ga_((1-y))N, AlGaN). Hereinafter the AlGaInN-based compoundsemiconductor forming the laminate structure body of the laser diode isreferred to as “the GaN-based compound semiconductor” in some cases, andthe AlGaInN-based compound semiconductor layer is referred to as “theGaN-based compound semiconductor layer” in some cases.

In the above-described preferable structure, the second compoundsemiconductor layer may have a superlattice structure in which p-typeGaN layers and p-type AlGaN layers are alternately laminated, and thethickness of the superlattice structure is 0.7 μm or less. When such asuperlattice structure is applied, while keeping a high refractive indexnecessary as a cladding layer, a series resistance component R of thelaser diode may be reduced to cause a reduction in operation voltage ofthe laser diode. The lower limit of the thickness of the superlatticestructure may be, for example, but not exclusively, 0.3 μm, and thethickness of the p-type GaN layer forming the superlattice structure maybe, for example, within a range from 1 nm to 5 nm both inclusive, andthe thickness of the p-type AlGaN layer forming the superlatticestructure may be, for example, within a range from 1 nm to 5 nm bothinclusive, and the total layer number of the p-type GaN layers and thep-type AlGaN layers may be for example, within a range from 60 layers to300 layers both inclusive. Moreover, the second electrode may bearranged on the second compound semiconductor layer, and a distance fromthe active layer to the second electrode may be 1 μm or less, preferably0.6 μm or less. When the distance from the active layer to the secondelectrode is determined in such a manner, the thickness of the p-typesecond compound semiconductor layer with high resistance may be reduced,and a reduction in operation voltage of the laser diode may be achieved.In addition, the lower limit of the distance from the active layer tothe second electrode may be, for example, but not exclusively, 0.3 μm.Moreover, the second compound semiconductor layer may be doped with1×10¹⁹ cm⁻³ or over of Mg, and the absorption coefficient of the secondcompound semiconductor layer for light with a wavelength of 405 nm maybe at least 50 cm⁻¹. The atomic concentration of Mg is derived from sucha material property that the maximum hole concentration is displayedwhen the atomic concentration of Mg is 2×10¹⁹ cm⁻³, and the atomicconcentration of Mg is a result by designing the maximum holeconcentration, that is, the specific resistance of the second compoundsemiconductor layer to be minimized. The absorption coefficient of thesecond compound semiconductor layer is determined only to minimize theresistance of the laser diode device, and as a result, the absorptioncoefficient of light into the active layer is typically 50 cm⁻¹.However, to increase the absorption coefficient, the Mg doping amountmay be intentionally set to be a concentration of 2×10¹⁹ cm⁻³ or over.In this case, the upper limit of the Mg doping amount under thecondition that a practical hole concentration is obtained is, forexample, 8×10¹⁹ cm⁻³. Moreover, the second compound semiconductor layermay include an undoped compound semiconductor layer and a p-typecompound semiconductor layer in order from the active layer side, and adistance from the active layer to the p-type compound semiconductorlayer may be 1.2×10⁻⁷ m or less. When the distance from the active layerto the p-type compound semiconductor layer is determined in such amanner, internal loss may be reduced without reducing internal quantumefficiency, thereby a threshold current density at which laseroscillation starts may be reduced. The lower limit of the distance fromthe active layer to the p-type compound semiconductor layer may be, forexample, but not exclusively, 5×10⁻⁸ m. Further, the laser diode mayhave a ridge stripe structure, and the width of a ridge section in theridge stripe structure may be 2 μm or less, and a laminated insulatingfilm made of an SiO₂/Si laminate structure may be formed on both sidesof the ridge section, and a difference between the effective refractiveindex of the ridge section and the effective refractive index of thelaminated insulating film may be within a range from 5×10⁻³ to 1×10⁻²both inclusive. When such a laminated insulating film is used, even ifhigh power operation exceeding 100 mW is performed, a single fundamentaltransverse mode may be maintained. The lower limit of the width of theridge section may be, for example, but not exclusively, 0.8 μm. Further,the second compound semiconductor layer may be formed, for example, bylaminating an undoped GaInN layer (a p-side light guide layer), anundoped AlGaN layer (a p-side cladding layer), a Mg-doped AlGaN layer(an electronic barrier layer), a GaN layer (doped with Mg)/AlGaN layersuperlattice structure (a superlattice cladding layer) and a Mg-dopedGaN layer (a p-side contact layer) in order from the active layer side.Further, the beam emission half angle θ⊥ in a perpendicular direction oflaser light emitted from an end surface of the laser diode may be 25degrees or less, preferably 21 degrees or less. The lower limit of thebeam emission half angle θ⊥ may be, for example, but not exclusively 17degrees. The resonant length may be, for example, within a range from0.3 mm to 2 mm both inclusive. The band gap of a compound semiconductorforming the well layer in the active layer is desired to be 2.4 eV orover. Moreover, the wavelength of laser light emitted from the activelayer is desired to be within a range from 360 nm to 500 nm bothinclusive, preferably within a range from 400 nm to 410 nm bothinclusive. The above-described various configurations may be combined asnecessary.

In the invention, various GaN-based compound semiconductor layersforming the laser diode are formed in order on a substrate. In thiscase, as the substrate, in addition to a sapphire substrate, a GaAssubstrate, a GaN substrate, a SiC substrate, an alumina substrate, a ZnSsubstrate, a ZnO substrate, an MN substrate, a LiMgO substrate, a LiGaO₂substrate, a MgAl₂O₄ substrate, an InP substrate, a Si substrate or asubstrate formed by forming a base layer or a buffer layer on a surface(main surface) of any one of these substrates may be used. Moreover, asa method of forming various GaN-based compound semiconductor layersforming the laser diode, a metal organic chemical vapor depositionmethod (an MOCVD method, an MOVPE method), a molecular beam epitaxymethod (an MBE method), a hydride vapor phase epitaxy method in which ahalogen contributes transport or reaction, or the like may be used.

In this case, as an organic gallium source gas in the MOCVD method,trimethyl gallium (TMG) gas or a triethyl gallium (TEG) gas may be used,and as a nitrogen source gas, an ammonia gas or a hydrazine gas may beused. When a GaN-based compound semiconductor layer having n-typeconduction is formed, for example, as an n-type impurity (an n-typedopant), silicon (Si) may be added, and when a GaN-based compoundsemiconductor layer having p-type conduction is formed, for example, asa p-type impurity (a p-type dopant), magnesium (Mg) may be added.Moreover, in the case where aluminum (Al) or indium (In) is included asa constituent atom of the GaN-based compound semiconductor layer, atrimethyl aluminum (TMA) gas may be used as an Al source, and atrimethyl indium (TMI) gas may be used as an In source. Further, as a Sisource, a monosilane gas (a SiH₄ gas) may be used, and as an Mg source,a cyclopentadienyl magnesium gas, methylcyclopentadienyl magnesium orbiscyclopentadienyl magnesium (Cp₂Mg) may be used. In addition, as then-type impurity (the n-type dopant), in addition to Si, Ge, Se, Sn, C,Te, S, O, Pd or Po may be used, and as the p-type impurity (the p-typedopant), in addition to Mg, Zn, Cd, Be, Ca, Ba, C, Hg or Sr may be used.

The second electrode electrically connected to the second compoundsemiconductor layer having p-type conduction (or the second electrodeformed on the contact layer) preferably has a single-layer configurationor a multilayer configuration including at least one kind of metalselected from the group consisting of palladium (Pd), platinum (Pt),nickel (Ni), aluminum (Al), titanium (Ti), gold (Au) and silver (Ag).Alternatively, a transparent conductive material such as ITO (Indium tinoxide) may be used. On the other hand, the first electrode electricallyconnected to the first compound semiconductor layer having n-typeconduction preferably has a single-layer configuration or a multilayerconfiguration including at least one kind of metal selected from thegroup consisting of gold (Au), silver (Ag), palladium (Pd), aluminum(Al), titanium (Ti), tungsten (W), copper (Cu), zinc (Zn), tin (Sn) andindium (In), and, for example, Ti/Au, Ti/Al, or Ti/Pt/Au may be used.The first electrode or the second electrode may be formed by, forexample, a PVD method such as a vacuum deposition method or a sputteringmethod. The first electrode is electrically connected to the firstcompound semiconductor layer, and a mode in which the first electrode isformed on the first compound semiconductor layer, and a mode in whichthe first electrode is connected to the first compound semiconductorlayer with a conductive material layer or a conductive substrate inbetween are included. In a like manner, the second electrode iselectrically connected to the second compound semiconductor layer, and amode in which the second electrode is formed on the second compoundsemiconductor layer, and a mode in which the second electrode isconnected to the second compound semiconductor layer with a conductivematerial layer in between are included.

A pad electrode may be arranged on the first electrode or the secondelectrode to be electrically connected to an external electrode or acircuit. The pad electrode preferably has a single-layer configurationor a multilayer configuration including at least one kind of metalselected from the group consisting of titanium (Ti), aluminum (Al),platinum (Pt), gold (Au) and nickel (Ni). Alternatively, the padelectrode may have, for example, a multilayer configuration such as aTi/Pt/Au multilayer configuration or a Ti/Au multilayer configuration.

The invention is applicable to, for example, fields such as optical disksystems, the communications field, the optical information field,opto-electronic integrated circuits, fields of application of nonlinearoptical phenomena, optical switches, various analysis fields such as thelaser measurement field, the ultrafast spectroscopy field, themultiphase excitation spectroscopy field, the mass analysis field, themicrospectroscopy field using multiphoton absorption, quantum control ofchemical reaction, the nano three-dimensional processing field, variousprocessing fields using multiphoton absorption, the medical fields andthe bio imaging field.

In the first embodiment of the invention, the laser diode is driven by apulse current which is 10 or more times higher than the thresholdcurrent value I_(th), and in the second embodiment of the invention, thelaser diode is driven by a pulse voltage which is 2 or more times higherthan the threshold voltage value V_(th). As a result, an ultrashortpulse/ultra-high power laser diode emitting laser light having a lightintensity of 3 watts or over and a pointed peak with a half-value widthof 20 picoseconds or less is provided.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are circuit diagrams of a laser diode apparatus ofExample 1, and FIGS. 1C and 1D are schematic views of a rectangularpulse voltage applied to a laser diode.

FIG. 2 is a schematic sectional view of a laser diode of Example 1.

FIG. 3 is a graph illustrating results of internal loss and internalquantum efficiency of laser diodes with different distances d from anactive layer to a p-type AlGaN electronic barrier layer in the laserdiode of Example 1.

FIGS. 4A to 4D are illustrations of waveforms of laser light emittedfrom the laser diode of Example 1.

FIGS. 5A to 5D are illustrations of waveforms of laser light emittedfrom a GaAs-based laser diode.

FIG. 6 is a graph illustrating peak light powers of first light peaksand pointed peaks which are obtained by changing an applied pulsevoltage V₂ in the laser diode of Example 1 and the GaAs-based laserdiode.

FIG. 7A is an illustration of a light waveform measured by a fastphotodetector and a sampling oscilloscope and a typical example of agenerated first light peak (GP) in the laser diode of Example 1, andFIG. 7B is an illustration of results obtained by measuring thehalf-value width of the first light peak (GP) by a streak camera in thelaser diode of Example 1.

FIG. 8 illustrates a graph illustrating the waveform of an applied pulsevoltage in the laser diode of Example 1 in (A), graphs illustratingwaveforms of a voltage generated between both ends of a resistor of 0.5ohms corresponding to a current monitor in (B) and (C), and a graphillustrating a relationship between a pulse voltage V₂ and a currentI_(Op) flowing through a laser diode in (D).

FIG. 9 is an illustration of a spectrum and an NFP before generating thefirst light peak (GP) in the laser diode of Example 1.

FIGS. 10A and 10B are illustrations of a spectrum and an NFP aftergenerating the first light peak (GP) in the laser diode of Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will be described in detail below referring to theaccompanying drawings.

Example 1

Example 1 relates to methods of driving a laser diode according to afirst embodiment and a second embodiment of the invention.

As illustrated in FIG. 1A, a laser diode apparatus including anultrashort pulse/ultra-high power laser diode of Example 1 includes apulse generator 10 and a laser diode 20 driven by a driving pulse fromthe pulse generator 10. More specifically, the laser diode apparatusincludes the GaN-based laser diode 20 with an emission wavelength of 405nm and the high-power pulse generator 10 allowing the GaN-based laserdiode 20 to perform gain switching operation. In addition, the laserdiode apparatus includes a DC constant current power source 11, but asillustrated in FIG. 1B, the DC constant current power source 11 is notnecessarily included. In this case, the DC constant current power source11 has a known circuit configuration, and the pulse generator 10 mayhave a configuration in which a low-voltage pulse generator and ahigh-power voltage amplifier are combined together.

As illustrated in FIG. 1C, a voltage (a driving pulse) applied to thelaser diode 20 is a rectangular pulse voltage V₂ with a duration t_(p).In addition, the DC constant current power source 11 is included, so thevoltage applied to the laser diode 20 is a voltage obtained by addingthe rectangular pulse voltage V₂ with the duration t_(p) to a DC voltageV₁. In this case, the DC voltage V₁ is V₁=R×I₁+V₀≈V₀=3 volts which isdetermined from a current (value: I₁) supplied from the DC constantcurrent power source 11, internal resistance R of the laser diode 20 anda built-in potential V₀ of a p-n junction. However, wiring resistance,contact resistance between wiring and the laser diode 20, or the like isnot considered in this case. In the circuit configuration illustrated inFIG. 1B, as illustrated in FIG. 1D, a voltage applied to the laser diode20 is a rectangular pulse voltage V₂ with the duration t_(p).

The laser diode 20 is a laser diode having a ridge stripe type separatedconfinement heterostructure (SCH structure). More specifically, thelaser diode 20 is a GaN-based laser diode made of index guide typeAlGaInN developed for a Blu-ray optical disk system, and has a ridgestripe structure. As specifications of the laser diode 20, the absolutemaximum rated light power is 85 milliwatts during continuous driving and170 milliwatts during pulse driving (a pulse width of 7.5 nanosecondsand a duty ratio of 50%). Moreover, the standard value of an emissionwavelength is 405 nm, a threshold current value I_(th), (the standardvalue of a oscillation start current) is 40 milliamperes, and thestandard values of an emission angle parallel to an active layer (a beamemission half angle θ// in a horizontal direction) of laser lightemitted from an end surface of the laser diode 20 and an emission angleperpendicular to the active layer (a beam emission half angle θ⊥) of thelaser light are 8 degrees and 21 degrees, respectively. The laser diode20 is a high-power laser diode in which light confinement in a direction(a vertical direction) where compound semiconductor layers which will bedescribed later are laminated is weakened. Further, the resonant lengthis 0.8 mm.

A schematic sectional view of the laser diode 20 is illustrated in FIG.2. The laser diode 20 is arranged on a (0001) plane of an n-type GaNsubstrate 21, and includes a laminate structure body including a firstcompound semiconductor layer 30, an active layer 40 having a quantumwell structure and a second compound semiconductor layer 50, a firstelectrode 61 electrically connected to the first compound semiconductorlayer 30 and a second electrode 62 electrically connected to the secondcompound semiconductor layer 50. The first compound semiconductor layer30, the active layer 40 and the second compound semiconductor layer 50are made of a GaN-based compound semiconductor, specifically, anAlGaInN-based compound semiconductor. More specifically, the laser diode20 has a layer structure indicated by the following Table 1. In thiscase, compound semiconductor layers in Table 1 are listed in order ofdecreasing distance from the n-type GaN substrate 21. In addition, theband gap of a compound semiconductor forming a well layer in the activelayer 40 is 3.06 eV.

TABLE 1 Second compound semiconductor layer 50 P-type GaN contact layer(Mg doped) 55 P-type GaN (Mg doped)/AlGaN superlattice cladding layer 54P-type AlGaN electronic barrier layer (Mg doped) 53 Undoped AlGaNcladding layer 52 Undoped GaInN light guide layer 51 Active layer 40GaInN quantum well active layer (Well layer: Ga_(0.92)In_(0.08)N/barrierlayer: Ga_(0.98)In_(0.02)N) First compound semiconductor layer 30 N-typeGaN cladding layer 32 N-type AlGaN cladding layer 31

Moreover, a part of the p-type GaN contact layer 55 and a part of thep-type GaN/AlGaN superlattice cladding layer 54 are removed by an RIEmethod to form a ridge section 56 with a width of 1.4 μm. A laminatedinsulating film 57 made of SiO₂/Si is formed on both sides of the ridgesection 56. The SiO₂ layer is a lower layer, and the Si layer is anupper layer. In this case, a difference between the effective refractiveindex of the ridge section 56 and the effective refractive index of thelaminated insulating film 57 is within a range from 5×10⁻³ to 1×10⁻²both inclusive, more specifically 7×10⁻³. The second electrode (a p-typeohmic electrode) 62 made of Pd/Pt/Au is formed on the p-type GaN contactlayer 55 corresponding to a top surface of the ridge section 56. On theother hand, the first electrode (an n-type ohmic electrode 61) made ofTi/Pt/Au is formed on a back surface of the n-type GaN substrate 21.

The thickness of the p-type GaN/AlGaN superlattice cladding layer 54having a superlattice structure in which p-type GaN layers and p-typeAlGaN layer are alternately laminated is 0.7 μm or less, specifically0.4 μm, and the thickness of each p-type GaN layer constituting thesuperlattice structure is 2.5 nm, and the thickness of each p-type AlGaNlayer constituting the superlattice structure is 2.5 nm, and the totalnumber of the p-type GaN layers and the p-type AlGaN layers is 160layers. A distance from the active layer 40 to the second electrode 62is 1 μm or less, specifically 0.6 μm. Moreover, the p-type AlGaNelectronic barrier layer 53, the p-type GaN/AlGaN superlattice claddinglayer 54 and the p-type GaN contact layer 55 constituting the secondcompound semiconductor layer 50 are doped with 1×10¹⁹ cm⁻³ or over(specifically 2×10¹⁹ cm⁻³) of Mg, and the absorption coefficient of thesecond compound semiconductor layer 50 for light with a wavelength of405 nm is at least 50 cm⁻¹, specifically 65 cm⁻¹. Further, the secondcompound semiconductor layer 50 includes an undoped compoundsemiconductor layer (the undoped GaInN light guide layer 51 and theundoped AlGaN cladding layer 52) and a p-type compound semiconductorlayer in order from the active layer side, and a distance d from theactive layer to the p-type compound semiconductor layer (specificallythe p-type AlGaN electronic barrier layer 53) is 1.2×10⁻⁷ m or less,specifically 100 nm.

In the laser diode 20 of Example 1, the p-type AlGaN electronic barrierlayer 53, the p-type GaN/AlGaN superlattice cladding layer 54 and thep-type GaN contact layer 55, which are Mg-doped compound semiconductorlayers, overlap a light density distribution generated from the activelayer 40 and its surroundings as little as possible, thereby internalloss is reduced without reducing internal quantum efficiency. As aresult, a threshold current density at which laser oscillation starts isreduced. FIG. 3 illustrates results of internal loss α_(i) and internalquantum efficiency η_(i) determined by actually forming laser diodeswith different distances d from the active layer 40 to the p-type AlGaNelectronic barrier layer 53. It was obvious from FIG. 3 that when thevalue of d increased, the internal loss α declined, but when the valueof d reached a certain value or higher, hole injection efficiency intothe well layer declined, thereby electron-hole recombination efficiencyin the active layer declined, and the internal quantum efficiencydeclined. The value of d was designed as described above on the basis ofthe above results.

In a method of driving the laser diode of Example 1, the laser diode isdriven by a pulse current which is 10 or more times, preferably 20 ormore times, more preferably 50 or more times higher than a thresholdcurrent value I_(th). The value of the current is a value far exceedinga current value (a rated current) necessary to obtain a rated lightpower. Alternatively, in the method of driving the laser diode ofExample 1, the laser diode is driven by a pulse voltage which is 2 ormore times, preferably 4 or more times, more preferably 10 or more timeshigher than a threshold voltage value V_(th), or the laser diode isdriven by a voltage increased to a voltage inducing transverse modeinstability or higher. Moreover, the laser diode 20 of Example 1 or thelaser diode 20 forming a laser diode apparatus of Example 1 is driven bya pulse current which is 10 or more times, preferably 20 or more times,more preferably 50 or more times higher than the threshold current valueI_(th), or by a pulse current far exceeding the rated current.Alternatively, the laser diode 20 of Example 1 or the laser diode 20forming a laser diode apparatus of Example 1 is driven by a pulsevoltage which is 2 or more times, preferably 4 or more times, morepreferably 10 or more times higher than the threshold voltage valueV_(th), or by a voltage increased to a voltage inducing transverse modeinstability or higher. Alternatively, the laser diode 20 of Example 1 orthe laser diode 20 forming the laser diode apparatus of Example 1 emitsa first light peak and a second light peak following the first lightpeak. The first light peak has a light intensity of 3 watts or over,preferably 5 watts or over, more preferably 10 watts or over and ahalf-value width of 20 picoseconds or less, preferably 15 picoseconds orless, more preferably 10 picoseconds or less, and the second light peakhas an energy of 1 nanojoule or over, preferably 2 nanojoules or over,more preferably 5 nanojoules or over and a duration of 1 nanosecond orover, preferably 2 nanoseconds or over, more preferably 5 nanoseconds orover.

When a pulsing voltage illustrated in FIG. 1C was applied, lightwaveforms illustrated in FIGS. 4A to 4D were observed from the laserdiode 20 of Example 1 through the use of a fast photodetector and asampling oscilloscope. In this case, the specifications of the appliedpulsed voltage are as illustrated in Table 2. The vertical axis in eachof FIGS. 4A to 4D indicates a signal voltage obtained from the fastphotodetector, and an output signal of 500 millivolts corresponds to alight power of 10 watts.

TABLE 2 DC constant current I₁: 0.1 milliamperes Pulse width t_(p): 2nanoseconds Pulse recurrence frequency f: 100 kHz

As illustrated in FIG. 4A, when a pulse voltage V₂ was 4.6 volts, asingle light peak was obtained. As illustrated in FIG. 4B, when thepulse voltage V₂ was 8.1 volts, a plurality of light pulses caused bythe relaxation oscillation of the laser diode were generated. Asillustrated in FIG. 4C, when the pulse voltage V₂ increased, in the casewhere the pulse voltage V₂ was 14.3 volts, a plurality of sharp lightpulses with a half-value width of 50 picoseconds or less were generated,and then a wide light pulse with a duration of approximately 1nanosecond is superimposed thereon.

When the pulse voltage V₂ was 16 volts, as illustrated in FIG. 4D, asharp single light pulse (called a giant pulse (GP), and correspondingto the first light peak) with a half-value width of 20 picoseconds orless and a high peak energy (approximately 10 watts) was generated, andafter the first light peak, a plurality of light pulses with lowintensity and a wide light peak with a duration of 1 nanosecond or over(which is the second light peak with a duration of approximately 1.5nanoseconds) superimposed on each other were observed. The value of apulse current at that time was 0.4 amperes or over, specifically 1.6amperes, or the value of the pulse current was 1.4×10⁵ ampere/cm² interms of the value of the pulse current per cm² of the active layer (percm² of a junction region area), that is, in terms of current density(which is operation current density in ampere/cm²).

The same experiment was performed on a GaAs-based high-power laserdiode. The results are illustrated in FIGS. 5A to 5D. The drivingconditions were the same as those in the laser diode 20 of Example 1.When the pulse voltage V₂ increased, a plurality of light pulses causedby relaxation oscillation and a wide light peak with a duration of 1nanosecond following the plurality of light pulses were observed.However, a light pulse with a high peak value such as the first lightpeak observed in the laser diode 20 of Example 1 was not observed. Thegeneration of the first light peak (GP) is considered as a uniquephenomenon obtained by gain switching operation performed by a GaN-basedlaser diode.

FIG. 6 illustrates peak light powers of the first light peak or apointed peak obtained by changing the applied pulse voltage V₂. In theGaAs-based high-power laser diode, as indicated by “B”, a narrow lightpulse with a half-value width of 50 nanoseconds was generated, but thepeak light power of the light pulse was a simple increasing functionwith respect to a pulse voltage. On the other hand, in the laser diode20 of Example 1, as indicated by “A”, when the pulse voltage V₂ exceeded15 volts, the peak light power was pronouncedly increased to generatethe first light peak (GP). In other words, the laser diode 20 of Example1 generated a light peak power which was a few times, in some cases, onedigit higher than an AlGaAs-based laser diode in related art.

A light waveform measured by the fast photodetector and the samplingoscilloscope and a typical example of the generated first light peak(GP) are illustrated in FIG. 7A. The first light peak (GP) with a highpeak light intensity of 15 watts and the second light peak with 1nanojoule or over, specifically 1.1 nanojoules and a duration of 1nanosecond or over, specifically 1.5 nanoseconds following the firstlight peak were generated. The driving conditions at that time are asillustrated in the following Table 3. Moreover, when the half-valuewidth of the first light peak (GP) was measured by a streak camera, thehalf-value width was 20 picoseconds which was very narrow (refer to FIG.7B).

TABLE 3 DC constant current I₁: 0.1 milliamperes Pulse width t_(p): 2nanoseconds Pulse recurrence frequency f: 100 kHz Pulse voltage V₂: 45volts

To consider the mechanism of the generation of the first light peak(GP), the pulse voltage V₂ applied to the laser diode 20 of Example 1 (alaser diode different from the laser diode used for the experimentillustrated in FIGS. 5A to 5D) and a current I_(Op) flowing through thelaser diode were measured before and after the generation of the firstlight peak (GP). The current I_(Op) flowing through the laser diode wasdetermined by inserting a resistor of 0.5 ohms into the laser diode inseries and then measuring a voltage between both ends of the resistor.The applied pulse waveform was obtained by adding a pulse voltage (witha width of approximately 2 nanoseconds, the voltage V₂) as illustratedin FIG. 8A to a DC constant current I₁=0.1 milliamperes. The pulserecurrence frequency f at that time was 100 kHz. Moreover, the waveformof a voltage generated between both ends of the resistor of 0.5 ohmscorresponding to a current monitor is illustrated in FIGS. 8B and 8C. Arelationship between the pulse voltage V₂ obtained in such a manner andthe current I_(Op) flowing through the laser diode is illustrated inFIG. 8D. In the experiment, the first light peak (GP) was generatedaround V₂=11 volts, but a current-voltage characteristic was not changedmuch before and after V₂=11 volts, and the gradient of thecurrent-voltage characteristic was constant. Therefore, it wasconsidered that the current I_(Op) flowing through the laser diode didnot cause the generation of the first light peak (GP).

Spectrums and NFPs (Near Field Patterns) before and after the generationof the first light peak (GP) are illustrated in FIG. 9 and FIGS. 10A and10B. The driving conditions at that time were the same as thoseillustrated in Table 1. In the experiment, a laser diode devicedifferent from the laser diodes used in the experiment illustrated inFIGS. 5A to 5D and 8A to 8D was used, so in the case of the pulsevoltage V₂=20 volts, the first light peak (GP) was not generated. Whenthe spectrum at that time was examined, as illustrated in FIG. 9, anoscillation peak at 402 nm and an oscillation peak at 407 nm on a longerwavelength side were observed. It was obvious from the NFP that in atransverse mode, in addition to a fundamental, a high-order modecomponent was generated at both sides of the vertical direction. Whenthe pulse voltage was increased to the pulse voltage V₂=23 volts, thefirst light peak (GP) was generated. When the spectrum and the NFP atthat time were measured, as illustrated in FIG. 10A, the above-describedoscillation peak at 407 nm on the longer wavelength side was eliminatedafter the generation of the first light peak (GP), and an oscillationpeak was generated on a shorter wavelength side around 395 nm. In FIG.10A, the oscillation peak around 395 nm overlapped the oscillation peakat 402 nm. When the signal of the oscillation peak at 395 nm wasextracted by a bandpass filter to measure the NFP, as illustrated inFIG. 10B, such a change that in the transverse mode, the fundamentalwith a large width in the vertical direction was broadened by thegeneration of the first light peak (GP) was observed.

Therefore, it was considered that the laser diode 20 of Example 1performed such Q switching laser-like operation that the first lightpeak (GP) was generated by having an energy accumulation mechanismcaused by instability in the transverse mode. In other words, the laserdiode according to the embodiment of the invention was considered as again switching type laser diode including a Q switching laser-likefunction by having the energy accumulation mechanism caused byinstability in the transverse mode. Therefore, it was considered that ashort light pulse width of 20 picoseconds or less and a peak light powerof 3 watts or over (for example, 10 watts or over) which were notobtained in a gain switching laser diode in related art were obtained bythe Q switching mechanism effectively underlying with an increase incurrent pulses.

There was a slight difference in the pulse voltage V₂ at which the firstlight peak (GP) was generated between laser diodes, and when the valueof the DC constant current I₁ increased, the value of the pulse voltageV₂ at which the first light peak (GP) was generated also increased. Morespecifically, in the case of I₁=0.1 milliamperes, and I₁=3 milliamperes,the pulse voltages V₂ illustrated in the following Table 4 were obtainedas the value of the pulse voltage V₂ at which the first light peak (GP)was generated.

TABLE 4 DC constant current I₁ 0.1 mA   3 mA Laser diode-A 19 volts 40volts Laser diode-B 13 volts 26 volts Laser diode-C 10 volts 23 volts

As described above, in Example 1, the laser diode 20 was driven by apulse current which was 10 or more times higher than the thresholdcurrent value I_(th), or the laser diode 20 was driven by a pulsevoltage which was 2 or more times higher than the threshold voltagevalue V_(th). As a result, an ultrashort pulse/ultra-high power laserdiode emitting laser light having a light intensity of 3 watts or overand a pointed peak with a half-value width of 20 picoseconds or less wasobtained. Moreover, in the laser diode of Example 1, a laser diodeemitting laser light having a light intensity of 3 watts or over and apointed peak with a half-value width of 20 picoseconds or less as thefirst light peak (GP), and the second light peak having a high energy of1 nanojoule or over and a high broad energy even with a duration of 1nanosecond or over following the first light peak (GP) was obtained.

Although the present invention is described referring to the preferableexample, the invention is not limited thereto. The configuration andstructure of the laser diode described in the example, the configurationof the laser diode apparatus are examples, and may be modified asappropriate. Moreover, in the example, various values are indicated, butthe values are also examples. Therefore, for example, when thespecifications of a used laser diode are changed, the values are alsochanged.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A method of driving a laser diode, comprising the step of driving thelaser using a pulse voltage which is 2 or more times higher than athreshold voltage value.
 2. The method of driving a laser diodeaccording to claim 1, wherein the width of the pulse voltage is 10nanoseconds or less.
 3. The method of driving a laser diode according toclaim 1, wherein the value of the pulse voltage is 8 volts or over. 4.The method of driving a laser diode according to claim 1, wherein thelaser diode is a laser diode having a ridge stripe type separatedconfinement heterostructure.
 5. The method of driving a laser diodeaccording to claim 2, wherein the laser diode includes a laminatestructure body including a first compound semiconductor layer, an activelayer having a quantum well structure and a second compoundsemiconductor layer, a first electrode electrically connected to thefirst compound semiconductor layer, and a second electrode electricallyconnected to the second compound semiconductor layer, and the laminatestructure body is made of an AlGaInN-based compound semiconductor. 6.The method of driving a laser diode according to claim 3, wherein thelaser diode includes a laminate structure body including a firstcompound semiconductor layer, an active layer having a quantum wellstructure and a second compound semiconductor layer, a first electrodeelectrically connected to the first compound semiconductor layer, and asecond electrode electrically connected to the second compoundsemiconductor layer, and the laminate structure body is made of anAlGaInN-based compound semiconductor.
 7. The method of driving a laserdiode according to claim 1, wherein the laser diode includes a laminatestructure body including a first compound semiconductor layer, an activelayer having a quantum well structure and a second compoundsemiconductor layer, a first electrode electrically connected to thefirst compound semiconductor layer, and a second electrode electricallyconnected to the second compound semiconductor layer, and the laminatestructure body is made of an AlGaInN-based compound semiconductor. 8.The method of driving a laser diode according to claim 7, wherein thesecond electrode is arranged on the second compound semiconductor layer,and a distance from the active layer to the second electrode is 1 μm orless.
 9. The method of driving a laser diode according to claim 7,wherein the second compound semiconductor layer is doped with 1×10¹⁹cm⁻³ or over of Mg, and the absorption coefficient of the secondcompound semiconductor layer for light with a wavelength of 405 nm is atleast 50 cm⁻¹.
 10. The method of driving a laser diode according toclaim 7, wherein the second compound semiconductor layer includes anundoped compound semiconductor layer and a p-type compound semiconductorlayer in order from the active layer side, and a distance from theactive layer to the p-type compound semiconductor layer is 1.2×10⁻⁷ m orless.